1. Field of the Technology of the Invention
The present invention relates to an improvement in a semiconductor device wherein electrode construction and soldering layer construction to bond the semiconductor substrate onto a mounting means have an improved combination for attaining stronger bonding force and higher stability for long time of service.
2. Description of the Prior Art
Metal electrode layers which is provided on a surface of a semiconductor substrate, and surface of which is to be bonded with a solder layer onto a mount should fulfil such characteristics that:
(1) the metal electrode layer contacting the solder layer has a good wetting with the latter,
(2) in the soldering process, the ingredient of the metal electrode layer contacting the solder layer substantially does not melt into the solder layer, and the metal electrode layer hardly peels off from the semiconductor substrate,
(3) the metal electrode layer contacting the solder layer has a strong bonding force to the latter,
(4) the metal electrode layer is stable and has a high reliability against a long-time hard testing such as thermal fatigue testing, and
(5) undesirable compound between the solder layer and the metal electrode layer is hardly made in the soldering process.
On the other hand, the solder should have characteristics that:
(1) it is easy to bond therewith,
(2) the solder has a good wetting with the semiconductor substrate, making no voids,
(3) the solder has a good electric conductivity,
(4) the solder has a good thermal conductivity,
(5) the solder is less oxidized by atmospheric gas, and
(6) the solder has good durability against thermal fatigue.
That is, the solder for the semiconductor substrate bonding, must not only have good physical and chemical characteristics, but also provide the semiconductor device with good electrical and thermal characteristics. In other words it is to be noted that, in bonding a semiconductor substrate on a mounting means together with substances of the electrode layers the solder substance as such gives a great effect to the bonding. Accordingly, in the soldering, consideration on materials should be made, not only on the electrodes of the semiconductor substrate, but also from a all-over view as a combination with the solder substance. Especially for semiconductor devices to handle large electric powers like power transistors or like ones, it is important to select the materials by considering the brittleness after time lapse, in order to realize a satisfactory resistivity to thermal fatigue.
Hitherto, nickel-plated layers formed by electrolytic process or non-electrolytic process, or nickel layer formed by vacuum deposition process have been used as electrode layers to be bonded by solder. Nickel is superior for this purpose since it does not melt into the solder layer, but has a problem that it requires a heat treatment at a relatively high temperature in order to attain a strong bonding force to the semiconductor substrate. When a nickel layer is formed by a plating, it is necessary to carry out a troublesome pretreatment in order to get excellent adhesive force to substrate surface, and moreover the plated surface is likely to be contaminated by impurities in a plating bath. In order to obtain a good obmic contact, a predetermined impurity is necessary in a contact region of the substrate to form the nickel layer thereon. And therefore, there is a problem that a diffusing process is necessary to form the contact region with the impurity.
In order to utilize the good characteristics of nickel for soldering, composite layers of chromium-nickel alloy and nickel or composite layers of chromium, chromium-nickel alloy and nickel have been developed and known as multiple layer electrode. However, even with the uses of such multiple layer electrode, it is not sufficient for attaining a stable and uniform bondings between the semiconductor substrate and the mount. In other words, even though such multiple layer electrodes are usable for actual uses, there is a problem of oxidation of nickel surface of the multiple layer electrodes in a mass-production line, where there is some waiting time between the electrode formation and subsequent solder bonding for devices in some lots. Therefore, ordinarily, some suitable surface treatment has been made in order to avoid undesirable influence of oxidation of surface nickel layer during the wasting time and obtain uniform products. This means an increase of a step, the surface treatment, which necessitates another control of its condition; and even with such treatment, voids are often observed at soldering. In view of the above-mentioned discussion, it is desirable to omit such surface treatment which is immediately before the soldering. And it is further considered that to form the nickel layer as the surface layer (uppermost layer) as such induces many problems.